When
Michael Birch met an American brunette named Xochi at a college
bar in London, he wasn't just meeting his future wife. It
was also the beginning of a beautiful business.

Earlier
this year the couple sold their social-networking site, Bebo,
to AOL for $850 million. Their cut: about $600 million. An
impressive haul for anybody, but especially for Michael and
Xochi, who are just 38 and 36.

The
world's current crop of billionaires has plenty of money but
not much youth. The average age of the 1,125 people on Forbes'
list of the world's wealthiest is 61. It's impossible to predict
exactly who will replace them, but don't bet against people
like the Birches.

Or
32-year-old Tiger Woods. When the golf champion returns to
the links from a knee injury, he'll continue a financial rise
that could crown him the first billionaire to make his money
through sports. We predict it could happen as soon as 2011
because of Tiger's lucrative endorsement deals.

Deal
maker Roelof Botha is another to keep an eye on. The 34-year-old
venture capitalist at Sequoia Capital hit a grand slam with
an early investment in YouTube. The deal yielded a 65-fold
return for Sequoia, a storied firm that also employs current
billionaire Michael Moritz.

Hedge
fund manager Chase Coleman's age, 33, belies his exceptional
ability to manage money. His Tiger Global fund reportedly
returned over 70 percent after fees last year, a performance
over six times better than the hedge fund average. One estimate
pegged Coleman's earnings from his banner year as high as
$400 million.

Even
younger is the glamorous Nishita Shah. She's just 28 but is
one of the richest people in Thailand because of her stake
in her family's sprawling business empire. Last month we estimated
her net worth at $375 million. When the stunning heiress is
not poring over financial reports or gracing the pages of
Thai magazines, she's planning her upcoming fashion line.

The
most recent Forbes Celebrity 100 list estimated that 38-year-old
entertainer Tyler Perry banked $125 million last year. The
talented Perry, who produces, directs and acts in his creations,
earned more than Jennifer Aniston, Tom Cruise and Bon Jovi
combined. Building a billion-dollar fortune in the entertainment
business is extraordinarily rare but certainly not impossible,
as evidenced by Steven Spielberg, worth an estimated $3 billion,
and George Lucas, worth an estimated $3.9 billion.

The
diversity of this group shows you can't predict what industry
the next billionaire will come from, but these people also
have something in common. Even though they've accumulated
more than enough money to retire in luxury, they aren't packing
up for the Hamptons.

"I'm
not really sure I believe in retirement," says Michael
Birch. He worries he would be bored. He and Xochi are spending
more time with their two children since the sale of Bebo,
but they're also hunting for the next big consumer Internet
hit.

Michael's
returning his attention to BirthdayAlarm.com, a site the couple
founded in 2001 but couldn't focus on because of Bebo and
other ventures. "We're going back to give it some love,"
says Michael. The idea's simple Ч a birthday reminder site
and e-card seller Ч but the potential's big. The site already
has about 50 million registered members.

Elon
Musk is also young, successful and full of big plans. He pocketed
$22 million at 27 after selling the dot-com company Zip2.
He followed that by co-founding PayPal. When the online payment
processor was sold to eBay, Musk received nearly $170 million
in eBay stock. The value of the stake only increased as eBay
stock appreciated.

Now
37, Musk isn't splitting time between vacation homes but between
a group of ambitious new businesses. His company, SpaceX,
wants to provide cheap, reliable space travel to clients such
as governments and satellite companies. Even though SpaceX
is still working out the kinks in its launch vehicles, Musk
says the company has been profitable for 18 months.

What
does Musk think are the most important qualities for an entrepreneur
like himself? Not surprisingly, "drive" is one of
his answers. When Musk isn't learning rocket science at SpaceX,
he's working on a venture that makes electric sports cars
and another one that designs and installs solar panels.